Electrophotographic marking is a well known and commonly used method of copying or printing original documents. Electrophotographic marking is performed by exposing a light image representation of a desired document onto a substantially uniformly charged photoreceptor. In response to that light image the photoreceptor discharges so as to create an electrostatic latent image of the desired document on the photoreceptor's surface. Toner particles are then deposited onto the latent image so as to form a toner image. That toner image is then transferred from the photoreceptor onto a substrate such as a sheet of paper. The transferred toner image is then fused to the substrate using heat and/or pressure. The surface of the photoreceptor is then cleaned of residual developing material and recharged in preparation for the production of another image.
The foregoing generally describes a black and white electrophotographic printing machine. Electrophotographic printing can also produce color images by repeating the above process for each color of toner that is used to make the color image. For example, the charged photoreceptive surface may be exposed to a light image which represents a first color, say black. The resultant electrostatic latent image can then be developed with black toner particles to produce a black toner layer which is subsequently transferred and fused onto a substrate. The process can then be repeated for a second color, say yellow, then for a third color, say magenta, and finally for a fourth color, say cyan. If the toner layers are placed in a superimposed registration the desired composite color toner image is formed on the substrate.
The color printing process described above superimposes the various color toner layers directly onto a substrate. Another electrophotographic color printing system makes use of an intermediate transfer member. In such systems successive toner layers are transferred in superimposed registration from the photoreceptor onto the intermediate transfer member. Only after the composite toner image is formed on the intermediate transfer member is that image transferred and fused onto the substrate.
The most common developing materials are dry powder toners. Dry powder toners are comprised of not only toner particles but also of carrier granules. The toner particles triboelectrically adhere to the carrier granules until the toner particles are attracted onto the latent image. An alternative to dry powder developing materials are liquid developers. Liquid developers, also referred to as liquid inks, have a liquid carrier into which toner particles are dispersed. When developing with liquid developers both the toner particles and the liquid carrier are advanced into contact with the electrostatic latent image.
Some electrophotographic printing machine use both liquid developers and an intermediate transfer member. In such machines the transfer of a toner layer from the photoreceptor to the intermediate transfer member, hereinafter referred to as the first transfer, is usually performed such that a first nip pressure is developed by the contact of the photoreceptor with the intermediate transfer member. Furthermore, the transfer of the composite color toner image from the intermediate transfer member to the substrate, hereinafter referred to as the second transfer, usually takes place such that a second nip pressure is developed by the contact of the intermediate transfer member with a backup roller. During image transfer the substrate is inserted between the intermediate transfer member and the backup roller. The combination of the second nip pressure and heat from an external source causes the color toner image to fuse with the substrate.
For example, U.S. Pat. No. 5,047,808 entitled, "Image Transfer Apparatus Including A Compliant Transfer Member," issued 10 Sep. 1991 to Landa et al, describes an electrophotographic printing machine which develops first and second nip pressures as described above. Landa et al. teaches transferring a toner layer from an imaging bearing surface onto an intermediate transfer member at a first pressure which causes the intermediate transfer member to deform to a first degree, and then the transfer of the toner image from the intermediate transfer member onto a substrate at a second pressure which causes the intermediate transfer member to deform to a second degree, wherein the second pressure exceeds the first pressure by a first multiple, and wherein the second deformation degree exceeds the first deformation degree by a second multiple, and wherein the second multiple is substantially less than the first multiple.
Systems as taught in Landa et al. are advantageous since the low pressure first transfer provides for a good transfer of the toner layer onto the intermediate transfer member without excessive wear of the photoreceptor while the high pressure second transfer ensures good image transfer onto the substrate even when the substrate's surface is rough, such as with uncoated papers. However, the high second pressure during the second transfer can cause belt tracking problems since the substrate tends to steer the intermediate transfer member during transfer. This steering increases the wear of the intermediate transfer member and reduces the quality of the transferred image.
Therefore, transfusing systems which avoid belt tracking problems yet which provide good image transfer onto relatively rough substrates would be beneficial.
Various approaches have been devised to produce multicolor color copies. The following United States Patents may be useful references:
U.S. Pat. No. 3,392,667 Patentee: Cassel et al. Issued: Jul. 16, 1968
U.S. Pat. No. 3,955,530 Patentee: Knechtel Issued: May 11, 1976
U.S. Pat. No. 4,348,098 Patentee: Koizumi Issued: Sep. 7, 1982
U.S. Pat. No. 4,588,279 Patentee: Fukuchi et al. Issued: May 13, 1986
The disclosures of the above-identified patents may be briefly summarized as follows:
U.S. Pat. No. 3,392,667 discloses a plurality of print cylinders having gravure engravings on their peripheries. Powder feed hoppers having rotating brushes apply powder to the print cylinders. The powder images from the print cylinders are transferred to an offset roller in superimposed registration with one another. The resultant powder image is then transferred from the offset roller to paper or sheeting.
U.S. Pat. No. 3,955,530 discloses a color image forming electrophotographic printing machine. Different color developers are used to develop the latent images recorded on the photoreceptive drum. Each developed image is sequentially transferred to an intermediate transfer drum. A cleaning blade is used to clean the photoreceptive drum between developing different color developers. The complete image is transferred from the intermediate drum to a copy sheet.
U.S. Pat. No. 4,348,098 discloses an electrophotographic copying apparatus which uses a transfix system. In a transfix system, the developed image is transferred from the photoconductive member to an intermediate roller. The intermediate roller defines a nip with a fixing roller through which the copy sheet passes. The developed image is then transferred from the intermediate roller to a copy sheet. The developing unit of the copying apparatus may either be a dry or wet type.
U.S. Pat. No. 4,588,279 discloses an intermediate transfer member that has a dry toner image transferred thereto from the surface of a toner image forming member. The toner image is then transferred from the transfer member to a recording paper.